Slew rate refers to the rate of change of a signal voltage in a circuit. Slew rate control is used to slow down the rising edge of a source voltage signal (VDD) to avoid supply transients while switching. In conventional applications, a high side switch is used to control slew rate as a power down feature of a chip. For example, when an enable signal (EN) is high, the high side switch supplies VDD to the chip. When EN is low, the high side switch puts the chip into a power down mode.
In some of the conventional implementations, slew rate control for a high side switch uses a level shifter which controls the slew rate in one step. Level shift circuitry to control a high side switch does not have any variable control over the slew rate because the level shift circuitry can only control slew in one step during the time the level shift circuitry is activated. In particular, when a control voltage pulse is applied to the input of the level shifter, the level shifter merely shifts the input voltage to generate the output voltage, which typically controls the slew rate according to a substantially linear progression.
Other conventional implementations control slew rate using a switch in combination with the level shifter. In this implementation, the level shifter initially turns on to control the slew rate in a substantially linear manner, as described above. Subsequently, at a predetermined output voltage, a switch is closed to change the functionality of the level shifter to operate similar to an inverter. By operating similar to an inverter, the high side switch can turn on faster relative to the operation of the high side switch when the level shifter is operational. However, the operation of the level shifter as an inverter does not control the slew rate.
Both of these conventional implementations merely control the slew rate during the operation of the level shifter. In contrast, there is no slew rate control before the time the level shifter is operational. Similarly, there is no slew rate control after the time that the level shifter operation ends, regardless of whether or not the level shifter operates as an inverter.